A conventional safety belt device for vehicles includes a belt retractor, a safety belt, and a fastening mechanism including a flashboard and a buckle. When the flashboard is inserted into the buckle, the belt is automatically retracted by the belt retractor to make the belt tightly bind body of a passenger. However, some people would not like to use the safety belt for they feel that the belt is too tight and uncomfortable, especially when they only dress light clothing in summer. Moreover, the clothing of the passenger may be wrinkled by the belt. Furthermore, the belt tightly attached to body may influence the operation of the driver.